Implantable cardioverter defibrillators (ICDs) require high-reliability power sources that provide long device life and a short, consistent capacitor charge time. Both of these performance parameters can be improved by optimizing battery performance. By increasing the life of the ICD, patients will require fewer replacement surgeries, lowering the lifetime cost of therapy for the patient. Predictably consistent performance of the power source is highly desirable. ICDs marketed by Medtronic, St. Jude, and Guidant all use lithium silver vanadium oxide (SVO) battery chemistries and all display a significant increase in charge time, in some cases up to 500%, as the battery is depleted. There have been cases with some device models where charge times become unacceptably long before the battery reaches the elective replacement indicator (ERI) voltage. Fast charge time is of clinical importance, and a delay in the delivery of the shock has negative physiologic effects. Presently no battery system fully provides all the performance characteristics desired by clinicians at relatively low cost. The research will focus on the development of a high-performance rechargeable lithium battery that will be used in conjunction with a primary lithium cell to provide a hybrid battery system. This hybrid combination of two distinct battery systems has demonstrated superior performance in a defibrillator application and the development of a rechargeable lithium battery with high energy density and low self-discharge will provide a high-performance, low-cost alternative power source for implantable medical devices with medium-to-high energy requirements. The major aim of the research is to identify and evaluate the state of rechargeable lithium battery technology, identify and procure the best candidate materials and processes needed to produce suitable cells, develop the hybrid battery design and develop the tools to evaluate battery performance.